In the prior art, a heterodyne transceiver circuit changes a communication frequency by changing an output frequency of a first local oscillator. The transceiver circuit has two second local oscillators for receiving and transmitting whose output frequencies are fixed.
FIG. 11 is a block diagram of the heterodyne transceiver circuit in the prior art. This transceiver circuit has a transmitter circuit 901, receiver circuit 902, a first local oscillator 903, a second local oscillator for transmitting 904, a second local oscillator for receiving 905, a diplexer 907, and an antenna 908.
The transmitter circuit 901 has a modulator 911, a filter 912, a frequency converter for transmitting 913, a filter 914, and an amplifier 915. The modulator 911 modulates the output signal of the second local oscillator for transmitting 904 with a base band signal. The output signal of the modulator 911 is provided to the frequency converter for transmitting 913 after passing the filter for reducing unwanted frequency components. The frequency converter for transmitting 913 produces a converted signal having a transmitting frequency by using the output signal of the first local oscillator 903. The output signal of the frequency converter for transmitting 913 is amplified by the amplifier 915 after unwanted frequency components of the signal are reduced by the filter 914. The amplified signal passes through the diplexer 907 and radiates from the antenna 908.
The receiver circuit 902 has a low-noise amplifier 921, a first frequency converter 922, a filter 923, a second frequency converter 924, a filter 925, and a demodulator 926. A signal received by the antenna 908 passes a diplexer 907 and is an input to the low-noise amplifier 921. The output signal of the low-noise amplifier 921 is provided to the first frequency converter 922, which produces a first intermediate frequency using the output signal of the first frequency local oscillator 903. The output signal of the first frequency converter 922 is provided to the second frequency converter 924 after unwanted frequency components of the signal are reduced. The second frequency converter 924 converts the output signal of the filter 923 into a second intermediate frequency using the output signal of the second local oscillator for receiving 905. The second intermediate frequency signal is demodulated by the demodulator 926 after unwanted frequency components of the signal are reduced by the filter 925.
However, the above transceiver circuit in the prior art needs two second local oscillators for receiving and transmitting. Therefore, this transceiver circuit is not suited for a portable transceiver that should be compact in size. The transceiver circuit in the prior art has another disadvantage. It has a tendency to generate unwanted spurious components due to the output signal of the second local oscillator at the transmission mode, as both second local oscillators are enabled during communication operation.